Real-time Hardware-in-the-Loop (HIL) Simulation is a tool to test and/or optimize a piece of hardware, for example a controller, within a simulated system. HIL simulation is applicable in many industries such as aerospace, automotive, maritime, offshore, robotics and electronics and power systems during development, testing and troubleshooting systems where the physical system can be expensive and simulation systems can be utilized in place of the physical system. Typical real-time digital simulation employ simulation hardware using very fast processors in order to perform the simulated systems in real-time suitable for the hardware in the loop simulation. The processors required for existing technologies present the major expense of the device. HIL simulation is faster and safer from an equipment safety point of view than connecting the hardware into the real physical system. Moreover, this technique is useful in validating implementation of designed controllers in embedded systems. However, the degree of complexity and the size of the simulated system may need to be limited to assure the real-time simulation of a model, otherwise, multiple real-time simulator devices are required in order to split the large systems into smaller subsystems simulated in real-time, which implies more costs to provide the simulation.
Systems such as modern power systems can cover large geographic areas and are an example of an industry where HIL simulation can be of significant benefit in deployment, testing and maintenance. The large size of the simulated power networks models coupled with the need for small simulation time-steps imposes significant challenges for performing real-time simulations as it requires very fast parallel processing computers. In real-time digital simulation the simulation software and the real-world hardware must exchange data in every time step, this requires sophisticated and nontrivial interface design between the real and simulated worlds. When real-time digital simulator is interfaced to external hardware, the interface can often result in an inaccurate simulation and even instability due to issues such as amplifier bandwidth and delays. There is no unique interface algorithm that provides the best stable and accurate results for every simulated system, therefore, depending on the system to be simulated, an appropriate interface algorithm must be selected. As a consequence, HIL simulation with real-time digital simulation is infeasible where the hardware under test is geographically remote from the real-time simulator, such as in another city or even another country. In addition, although advances in development of fast processors such as digital signal processing (DSP) and FPGA technologies has helped address the challenges of simulating large power systems significant hardware and software expenses are incurred.
Waveform Relaxation (WR) based HIL simulation is considerably cost effective compared to the state of the art technologies. Waveform relaxation is traditionally an iterative method of solving systems of ordinary nonlinear differential equations. WR based HIL simulation can also be used for the iterative simulation of systems, for a large system, the system can be divided into two or more different subsystems and each be simulated independent of the other(s). The results are exchanged when simulations end, and the simulations are repeated. If certain convergence criteria are met, after enough number of iterations, the simulation results show no more changes, and can be said to have converged.
A significant drawback of the WR based HIL simulation is the possible slow convergence of the simulation. The WR convergence speed highly depends on the tightness between the subsystems. If the hardware-under-test is loosely connected to the simulation, then only a few iterations are required to reach convergence. Another disadvantage of the WR based HIL simulation is the limited total simulation time. Available real-time HIL simulators exchange the data points at every simulation time step placing considerable processor and memory demands. However, there is usually no need for a long simulation period if the transient response of the HUT is of interest. Another shortcoming of the WR based HIL simulation is that it is inherently an off-line simulation, for example once a step change is applied to the simulated system or to the parameters of the hardware, the results cannot be immediately followed. Instead, the WR should be allowed to converge in a course of iterations. The main question regarding the WR based HIL simulation is whether the simulation convergences for a specific system. The tight connection of the subsystems can slow down the convergence speed and the simulation may sometimes not converge. In addition, the WR simulation can encounter instabilities when applied to the HIL simulation with the presence of noise and analog-digital converters inaccuracies even for theoretical stable regions of simulation. The instabilities can occur when the simulation and hardware subsystems are connected fairly tight and can impact the accuracy and speed of the simulation.
Accordingly, systems and methods that enable improved hardware-in-loop (HIL) simulations remain highly desirable.
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